Parent object to mirrored side of object
That can be done with some constraints:

Place an empty has child of the original cube. This is to place Suzanne relatively to the cube.
Then add two &quot;transformation&quot; constraints to Suzanne, with the above empty as target.

Presuming the mirror is on X.
One constraint for the locations, inverting X source (maps -1/1 to 1/-1).
One constraint for the rotations, inverting Y and Z source either (the same way).